This invention relates to an apparatus for welding together sheets of plastic, particularly but not exclusively for welding and sealing plastic bags containing waste or hazardous material.
When processing or handling small quantities of radioactive material it is known to use a glove box, the glove box shielding the operator from the radiation, and incorporating gloves mounted in ports in the wall of the box so the operator can manipulate objects within the box. Objects may be transferred out of the box through a port into a plastic tube or sleeve, and the plastic sleeve then sealed to form a bag enclosing the objects. Hence objects can be removed while remaining sealed from the environment. A similar procedure may be used when removing objects from a hot cell in which they are handled by a manipulator. The use of dielectric welding (which may also be referred to as radio frequency heating or high frequency heating) for welding such a plastic tube or bag has been known for many years. In this process the two films of thermoplastic material (such as polyvinyl chloride) are positioned between opposed electrodes (or one electrode and a base plate), the electrodes are pressed together, and a radio frequency voltage is applied between the electrodes. This process is applicable to materials which have a significant dielectric loss index, for example greater than 0.2, at the radio frequency. This may be between 1 MHz and 200 MHz, usually between 10 MHz and 100 MHz, although in practice it may be constrained by regulations, for example to be either 27.12 MHz or 40.68 MHz. However, such tools as used hitherto are no longer acceptable because the operating frequency varies with the electrical load during the welding process, and so the tool transmits radio waves into the surroundings at frequencies other than those permitted for dielectric welding.
According to the present invention there is provided a dielectric welding apparatus for welding a plastic sleeve, the apparatus comprising a solid state radio frequency generator, a matching network, and a welding tool, the tool comprising a frame with a handle, the frame defining a slot in which are opposed welding electrodes, the slot being open at one end and at the other end communicating with a wider aperture, and means for urging the electrodes together, the welding electrodes being connected via the matching network to the generator.
Preferably the electrodes are coated with an electrically insulating material that is not dielectrically heated, for example PFA (perfluoro alkoxyalkane). Preferably the electrodes are readily removable, so they can be replaced; this may be because the electrically insulating coating becomes worn, or alternatively the electrodes might be replaced by electrodes of a different shape, though preferably of the same area.
Preferably the radio frequency generator and the matching network are in a cabinet, and the welding tool is connected to the cabinet by a coaxial cable. The cable is preferably of a length that is a whole number of quarter wavelengths of electromagnetic radiation along the cable at the operating frequency. The means for urging the electrodes together may for example be a pneumatic cylinder, and may be under trigger control. The coaxial cable is preferably linked to the cable or tube carrying the power for urging the electrodes together, for example the pneumatic tube if a pneumatic cylinder is involved, and both the coaxial cable and the power supply cable or tube are fed into the frame of the tool opposite the end at which the slot is open. This helps balance the tool during use.
In the preferred embodiment the electrodes are urged together by a pneumatically actuated mechanism, and the cabinet incorporates a compressor and a reservoir for compressed air. The compressor is arranged to switch on if the pressure is below a lower operating limit, and to switch off when the pressure reaches an upper operating limit. However, if the pressure drops to below a threshold (below the lower operating limit) this indicates a significant leak in the pneumatic system, and the compressor is switched off.